Mobile communication devices including cell phones as representative examples and devices for their base stations, network-associated electronic devices, such as a server and a router, large-size computers, and the like are demanded to transmit and process a large amount of data with low loss at a high speed. For transmitting and processing a large amount of data at a high speed; an electric signal having a high frequency is needed. However, basically, the higher the frequency, the more likely the electric signal attenuates. That is, the electric signal having a higher frequency has properties such that the output is likely to weaken in a shorter transmission distance to cause large loss. Therefore, for meeting the demands for the above transmission and processing of data with low loss at a high speed, the printed circuit board mounted on the device for transmission and processing of data must be improved in its properties to further reduce the transmission loss, particularly, transmission loss in a high frequency band.
For obtaining a printed circuit board having a low transmission loss, substrate materials using a fluororesin having a low relative permittivity and a low dielectric loss tangent have conventionally been used. However, the fluororesin generally has a high melt temperature, a high melt viscosity, and relatively low flowability, and therefore has a problem in that high-temperature and high-pressure conditions must be employed in the pressing for the fluororesin. In addition, the fluororesin also has a problem in that, when applied to the use of high-multilayer printed circuit board for use in the above-mentioned communication devices, network-associated electronic devices, large-size computers, and others, the fluororesin is unsatisfactory in the workability, dimensional stability, and adhesion to the metal plating.
Therefore, as a substitute for the fluororesin, resin materials for printed circuit board which meet the requirements of the high frequency application are being studied. Of these resin materials, the use of polyphenylene ether known as one of the resins having the most excellent dielectric properties among the heat-resistant polymers has attracted attention. However, like the fluororesin, the polyphenylene ether is a thermoplastic resin having a high melt temperature and a high melt viscosity. Therefore, when applied to the use of printed circuit board, for lowering the melt temperature and melt viscosity of the resin so that lower-temperature and lower-pressure conditions can be employed in the pressing, or for imparting to the resin a heat resistance at the melt temperature of polyphenylene ether (230 to 250° C.) or higher, a resin composition using polyphenylene ether and a thermosetting resin in combination has conventionally been used.
For example, a resin composition using a polyphenylene ether and an epoxy resin in combination (see Patent document 1), a resin composition using a polyphenylene ether and a bismaleimide in combination (see Patent document 2), a resin composition using a polyphenylene ether and a cyanate ester in combination (see Patent document 3), a resin composition using a polyphenylene ether, a styrene-butadiene copolymer or polystyrene, and triallyl cyanurate or triallyl isocyanurate in combination (see Patent documents 4 and 5), a resin composition using a polyphenylene ether and a polybutadiene in combination (see Patent documents 6 and 7), a resin composition obtained by preliminarily-reacting modified polybutadiene having a functional group, such as a hydroxyl group, an epoxy group, a carboxyl group, or a (meth)acryl group, and bismaleimide and/or cyanate ester (see Patent document 8), a resin composition using a polyphenylene ether having added or grafted thereon a compound having an unsaturated double bond-containing group and the above triallyl cyanurate, triallyl isocyanurate, or polybutadiene in combination (see Patent documents 9 and 10), a resin composition using a reaction product of a polyphenylene ether and an unsaturated carboxylic acid or unsaturated acid anhydride and the above bismaleimide in combination (see Patent document 11), a resin composition using a low molecular-weight (oligomer) type polyphenylene ether oligomer having an unsaturated double bond-containing group at the end thereof and polybutadiene or a styrene-butadiene copolymer in combination, and a resin composition using the above polyphenylene ether resin composition and an inorganic filler in combination (see Patent document 12) have been proposed. These patent documents disclose that, for removing the above-mentioned disadvantages of the thermoplastic resin while maintaining the low transmission loss of the polyphenylene ether, it is preferred that the cured resin does not have many polar groups.    [Patent Document 1] Japanese Patent Application Laid-Open No. 58-69046    [Patent Document 2] Japanese Patent Application Laid-Open No. 56-133355    [Patent Document 3] Japanese Published Examined Application No. 61-18937    [Patent Document 4] Japanese Patent Application Laid-Open No. 61-286130    [Patent Document 5] Japanese Patent Application Laid-Open No. 3-275760    [Patent Document 6] Japanese Patent Application Laid-Open No. 62-148512    [Patent Document 7] Japanese Patent Application Laid-Open No. 59-193929    [Patent Document 8] Japanese Patent Application Laid-Open No. 58-164638    [Patent Document 9] Japanese Patent Application Laid-Open No. 2-208355    [Patent Document 10] Japanese Patent Application Laid-Open No. 6-184213    [Patent Document 11] Japanese Patent Application Laid-Open No. 6-179734    [Patent Document 12] Japanese Patent Application Laid-Open No. 2005-105061